1. Field of the Invention
The present invention relates to a storage means access control method applied to a numerically-controlled (hereinafter referred to as "NC") system for a machine tool or the like, which has a first storage means and a second storage means having an access time longer than that of the first storage means, to control access to the second storage means. The present invention also relates to such an NC system.
2. Related Background Art
Recently, with the progress of numerically-controlled machine tools, there has been an increasing demand for NC systems. A typical existing NC system contains a CPU( e.g., a microprocessor), and has a part that analyzes an NC program and sends a control signal to a servomotor for each control axis, and a part that executes sequence control (such as, tool change). Such an NC system can be realized by using a general-purpose computer. If an NC system is realized by using a general-purpose personal computer (hereinafter referred to as "personal computer"), which is now available at low cost, it is possible to reduce the cost of the NC system. The term "personal computer" herein used means a small-sized general-purpose computer for personal use (for a single user), and it includes FA personal computers (i.e., personal computers for factory use) in addition to so-called OA personal computers (i.e., personal computers for business use).
A special-purpose NC system is required to exhibit a high level of real-time performance. Therefore, all necessary data, including data concerning an NC program to be executed by the NC system and a program and data concerning another task to be performed in parallel by multitask processing, is stored in a main memory comprising a semiconductor memory. In an NC system using a personal computer, the personal computer contains a magnetic disk unit,(e.g., a hard disk unit), in addition to a main memory comprising a semiconductor memory. In multitask processing, a task other than a task of processing an NC program may make access to the magnetic disk. For example, there are cases where an NC program other than one that is being executed is input, edited and saved to the magnetic disk, and where an NC program other than one that is being executed is read out from the magnetic disk to modify it. When the magnetic disk is accessed as described above, the processing executed by the CPU is monopolized by the disk access. Therefore, other processing cannot be executed during the disk access.
Thus, in a personal computer, the processing executed by the CPU is monopolized by the access to a magnetic disk unit or the like; therefore, no other task can be carried out during the disk access. Accordingly, a task being carried out to process an NC program may be suspended, or a wait may occur between an NC instruction which is being executed and a subsequent NC instruction. If such a wait occurs during control of a machine tool, for example, the tool temporarily stops on the machining path, and it becomes impossible to maintain the continuity in the motion of the tool. This has an adverse effect on the machining process, such as a feed mark made on the machined surface of a workpiece.